This invention relates to a color optical filter element for optical usages, such as a charge coupled device or color liquid crystal display.
To obtain color image recording using solid state image devices such as charge coupled devices, optical filters in a multicolor stripe or mosaic form are employed. A plurality of filter elements are employed generally red, green and blue or cyan, yellow and magenta. However, in some instances, fewer or additional colors may be employed.
A number of processes are disclosed in the art for preparing such filters. One widely used method of forming such filters may be broadly described as consisting of the steps of coating a photoresist, exposing incremental portions of the photoresist by the use of a mask or other means; development of the exposed portions, applying a color or rendering the photoresist colorless and then removing either the exposed or unexposed portion of the photoresist. These steps are then repeated for each color desired.
Most of the procedures employed in forming such filters on charge coupled devices (CCD) pose a number of problems. To begin with, the process itself is generally complicated and difficult. In addition, to be a useful process, the steps must conform to the requirements of the clean room environment where CCDs are manufactured. Further, the process involved must not involve any conditions which would be detrimental to the CCD and, of course, problems such as polymer or dye stability and adhesion of the filter layers to the CCD must also be addressed.
U.S. Pat. No. 4,311,733, issued Jan. 19, 1982, is directed to a method for producing color filters, which method comprises the steps of coating on a substrate, for example, a solid-state imager, a mixture of the light-absorbing material such as a dye and a photosensitive material, for example, a polymer such as gelatin or polyvinyl alcohol with a cross-linking agent. Since the unexposed polymer is removed as a result of this process, the material is classified as negative working. One requirement for the dye is that it can be removed with an appropriate solvent and will not remain as part of the final filter element. The thus-coated layer is exposed through a predetermined pattern. The light-absorbing material serves to attenuate the exposure from secondary reflections from the substrate that causes line distortion. The unexposed areas of the photosensitive material is removed with an appropriate solvent and the light-absorbing material from the photosensitive material which remains on the substrate is also removed either simultaneously with the removal of the unexposed photosensitive material or in a separate step. The last step involves the dyeing of the remaining photosensitive material adhering to the substrate to the desired color.
U.S. Pat. No. 4,383,017, issued May 10, 1983, is directed to a method for manufacturing a stripe filter about 10 micrometers or wider for optical uses particularly in a television camera. The method comprises a repeated series of steps which include coating a support with a mixture of a dye, dissolution accelerator and a photosensitive polymeric material such as dichromated gelatin. The coating is exposed through a mask, thereby forming a series of insoluble stripes in the exposed areas. Thus, the polymeric material can be classified as a negative photoresist. The insoluble material is then removed and the steps repeated using a polymeric coating composition with a different color each time. More specifically, the coating composition includes a photosensitive hardener, which upon exposure, as by ultraviolet rays, accelerates the reaction among the molecules of the coating material performing a cross-linking effect.
It is known in the art to mix dyes with photoresists prior to coating the photoresist on a substrate in etching processes in the manufacture of microcircuitry units. The concentration of the dye additive provides an antihalation effect or reduction of the standing wave effect. Reflected light from the substrate serves to distort the final patterned image, decreasing resolution. The dye employed must absorb in the wavelength of the exposing wavelength in order to function. After the etching step is carried out, the photoresist is removed. Thus, it will be seen that the dye in the photoresist performs a function only during exposure of the photoresist to provide anti-reflection effect by employing a uniformly dyed layer with a single color at a low dye density and which layer is removed prior to use of the element on which it is coated. A description of this may be found, for example, in U.S. Pat. No. 4,377,631, issued Mar. 22, 1983, and Neureuther and Dill, "PHOTORESIST MODELING AND DEVICE FABRICATION APPLICATIONS", PROC. MICROWAVE RESEARCH SYMP. XVIII, NEW YORK, APRIL 1974.
Japanese Patent Application No. 58/100107, application date Dec. 10, 1981, published June 14, 1983, is directed to a method for forming a filter employing a dye dispersion in a silver halide emulsion. A layer of dye dispersion, e.g., dye developer particles in a very fine grain silver halide emulsion is formed on a support. The emulsion is exposed to light through a mask. Where the light strikes the emulsion, developable silver is formed. Upon contact with a tanning developer, cross-linking of the gelatin occurs in the generation areas of exposure by the action of the silver and the oxidized developer. The unexposed, and therefore un-cross-linked, emulsion is removed by washing. The cross-linked areas are then subjected to bleaching and fixing which removes the silver, leaving only the dye-dispersion in the cross-linked gelatin. In spite of the "positive-type" terminology employed, the patent is employing a system which is essentially a "negative photoresist" since cross-linking occurs in the areas of exposure and the unexposed areas are washed away.
In employing dyes in positive photoresists, the prior art has been concerned with the effect of the dye on the entire system since the addition of the dye constitutes "foreign" matter in the photoresist composition which could effect the functioning of the photoresist. More specifically, such foreign matter could interfere with the dissolution of the photoactive compound. To avoid any adverse effects, the levels of the absorbing dyes employed were kept low. Still further, many dyes would be incompatible with the photoresist composition, forming a separate phase.